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Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| Unstable quasi g-modes in rotating main-sequence stars
This paper studies the oscillatory stability of uniformly rotatingmain-sequence stars of mass 3-8 M_ȯ by solving the linearizednon-adiabatic, non-radial oscillation equations with a forcing term andsearching for resonant response to a complex forcing frequency. By usingthe traditional approximation, the solution of the forced oscillationequations becomes separable, whereby the energy equation is madeseparable by approximation. It is found that the κ-mechanism inrotating B-stars can destabilize not only gravity- or pressure modes,but also a branch of low frequency retrograde (in corotating frame)oscillations in between the retrograde g-modes and toroidal r-modes.These unstable quasi-g (or "q-modes") hardly exhibit rotationalconfinement to the equatorial regions of the star, while theoscillations are always prograde in the observer's frame, all incontrast to g-modes. The unstable q-modes occur in a few narrow periodbands (defined by their azimuthal index m) and seem to fit theoscillation spectra observed in SPB stars rather well. The unstableq-mode oscillation spectrum of a very rapidly rotating 8 M_ȯ starappears similar to that of the well-studied Be-star μ Cen. Theunstable q-modes thus seem far better for explaining the observedoscillation spectra in SPB-stars and Be-stars than do normal g-modes.
| Stellar and circumstellar activity of the Be star mu Centauri. III. Multiline nonradial pulsation modeling
After the description and time series analysis of the variability of thecircumstellar and stellar lines, respectively, in Papers I and II ofthis series, this paper sets out to model the stellar variability interms of multi-mode nonradial pulsation (nrp), but also adds another 109echelle spectra to the database, obtained in 1999. While thenear-circumstellar emission has faded further, the six periods and theassociated line profile variabilities (lpv) have remained unchanged. Forthe modeling, P1 of the periods P1- P4close to 0.5 day, and P5 of the two periods P5 andP6 near 0.28 day were selected, because they have the largestamplitude in their respective groups, which are characterized by theirown distinct phase-propagation pattern. Permissable ranges of mass,radius, effective temperature, projected equatorial rotation velocity,and inclination angle were derived from calibrations and observationsavailable in the literature. A total of 648 different combinations ofthese parameters were used to compute a number of trial series of lineprofiles for comparison with the observations. Next to reproducing theobserved variability, the primary constraint on all models was that thetwo finally adopted solutions for P1 and P5 had tobe based on only one common set of values of these quantities. This was,in fact, accomplished. Townsend's \cite{1997MNRAS.284..839T} code Brucewas deployed to model the pulsational perturbations of the rotationallydistorted stellar surface. With the help of Kylie, from the same author,these perturbations were converted into observable quantities. The localflux and the atmosphere structure were obtained from a grid of Atlas9models with solar metallicity, while the formation of 5967 spectrallines was calculated with the LTE code of \cite{bht}. An initial coarsegrid of models using all these ingredients was computed for all 12 nrpmodes with l <= 3 and m !=q 0. Comparison with the observedvariability of C ii 4267, which is the best compromise betweencontamination by circumstellar emission and significance of thevariability, yielded (l = 2, m = +2) for P1 (and, byimplication, P2-P4) and (l = 3, m = +3) forP5 (and P6) as the best matching nrp modes. At 9Msun / 3.4 Rsun and 440 km s-1,respectively, the mass-to-radius ratio and the equatorial velocity areon the high side, but not in fundamental conflict with establishedknowledge. The photometric variations of all six modes combine at mostto a maximal peak-to-peak amplitude of 0.015 mag, consistent with thenon-detection of any of the spectroscopic periods by photometry. Withoutinclusion of additional physical processes, present-day linear nrpmodels are fundamentally unable to explain major red-blue asymmetries inthe power distribution, which however seem to be limited to only somelines and the modes with the highest amplitudes. Nevertheless, the modelreproduces very well a wide range of observed details. Most notableamong them are: (i) Although all modeling was done on the residuals fromthe mean profiles only, the mean spectrum predicted by the model closelyfits the observed one. (ii) Dense series of high-quality spectraobtained as early as 1987 and as recently as 1999, publishedindependently but not included in the modeling efforts of this paper,are matched in great detail by the multiperiodic nrp model. As inomega CMa, the inferred modes are retrograde in thecorotating frame and in the observer's frame appear prograde onlybecause of the rapid rotation. This has implications for models of theejection of matter during line emission outbursts, which in mu Cen arecorrelated with the beating of modes in the 0.5-d group of periods. Thelength of the corotating periods as well as the horizontal-to-verticalvelocity amplitude ratios suggest a g-mode character. Based onobservations collected at the European Southern Observatory at La Silla,Chile, ESO proposal Nos.~55.D-0502, 56.D-0381, 58.D-0697, 63.H-0080.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| 53 Persei: a slowly pulsating B star
A new investigation of the behaviour of the star 53Per was performed.New photometric observations allow to confirm two dominant periodsp_1=2.16d, p_2= 1.67d. A third one p_3= 3.64d is detected. These longperiod variations are present in our radial velocities data and in oldequivalent width data. The amplitude of the p_1 = 2.16d period increasesfrom 1977 to 1991, while the amplitude of the 1.67d period remainsconstant. Considering the observational characteristics of the starsince 1997, 53Per clearly belongs to the Slowly Pulsating B stars groupas defined by Waelkens (1991) and North & Paltani (1994).spectroscopic observations performed at the Haute Provence Observatory
| HD 37151: A new 'slowly pulsating B star'
We report the discovery of non-radial pusations with at least 5 periodsin the B8V star HD 37151, a probable member of the Orion OB1association. This result is based on 465 photometric measurements in 7colors spanning 12 years. In addition, 30 high-resolution spectra weretaken in the Mg II lambda 4481 region: they show a small projectedrotational velocity and slight variations of the line profiles where atleast two out of the five photometric periods can be identified.Although this star was once classified B8Vp(Si), all available data showit is a normal star instead. Therefore, this object in not anotherexample (after ET And) of a pulsating silicon star, but entirely matchesthe characteristics of Waelkens' 'Slowly Pulsating B stars' (SPB). Itoffers the interest of being the second coolest member of this class,helping to define empirically the cool border of the related instabilitydomain. The ratio of color to visual amplitudes A(sub(u-v)/Avof the known SPB stars is nicely correlated with Teff, ingood agreement with theoretical expectations for non-radial pulsationswith order less than or = 2.
| The Opacity Mechanism in B-Type Stars - Part Two - Excitation of High-Order G-Modes in Main Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.265..588D&db_key=AST
| On non-radial oscillations of B-type stars
The excitation mechanisms behind the slow pulsations - with periods ofthe order of one day - of OB stars are discussed on the basis ofnumerical computations. Nonradial, non-adiabatic stability analysis isperformed on a set of evolutionary models with masses between 5.2 and12.1 solar masses. The long-period oscillations in stellar models belowthe newly recovered Beta Cephei region are found to be excited by theiron bump in the newly available OPAL opacities. For more massive starsexhibiting oscillations with periods of the order of one day, however,another excitation mechanism must be sought.
| The 71st Name-List of Variable Stars
Not Available
| Vibrational stability of an evolved 5-solar-mass star towards higher-order g-modes
The vibrational stability towards low-degree higher-order nonradialg(+)-modes has been studied for an evolutionary model of a 5 solar massstar near the end of central hydrogen burning, for the oscillations withperiods in the range of one to three days found in mid-B variables. Thestability analysis is performed on the basis of the quasi-isentropicapproximation. The effects of convection are neglected. The model withlog10 T(eff) = 4.17 and log10 L/L(solar) = 2.93 is shown to bevibrationally unstable towards the second-degree mode g(+)38 with aperiod of 1.57 days. The main cause of the driving is thekappa-mechanism in the He II ionization zone.
| Slowly pulsating B stars
Photometric data obtained during several years of observations of sevenB-type stars are analyzed, including HD 74195 (Omicron Velorum), HD74560 (HD 3467), HD 123515 (HR 5296), HD 143309, HD 160124, HD 177863(HR 7241), and HD 181558 (HR 7339). Results indicate that all sevenstars are multiperiodic variables with periods of the order of days. Twoperiods were identified for HD 177863, three periods for HD 74560 and HD181558, four periods for HD 123515, five periods for HD 74195, sixperiods for HD 143309, and eight periods for HD 160124. Themultiperiodicity and the amplitude behavior of these stars point towardpulsation in high-radial-order g-modes in the stars. It is suggestedthat these stars form a distinct group of early-type variables, whichare named here 'slowly pulsating B stars'.
| Stromgren photometry of open clusters. I - NGC 6281, NGC 6405
Stromgren indices for A-type and B-type stars in the southern openclusters, NGC 6281 and NGC 6405, are presented. Their membership isdiscussed and the reddening of the clusters is estimated. It is shownthat shifting of the main sequence to the standard relation of Crawford(1975, 1978, 1979) yields a reddening of E(b-y) = 0.120 for NGC 6281 anda reddening of E(b-y) = 0.110 for NGC 6405. The occurrence of a CP1(Am)and a Delta-Sct star in NGC 6405 is considered. The results of theStromgen photometry are given in tables.
| Photometric variability of mid-B stars
An extended systematic study was carried out of photometric variationsof B-type main sequence stars. The campaign was performed using theseven-color instrument at La Silla. The survey covered 30 stars andshowed that the objects could be typified by the 53 Per variables. Theobserved variabilities may arise from pulsations in nonradial g-modeswith high radial wavenumbers. The motions detected occur in the outerstellar layers, i.e., in the second He ionization zone, a situation withsignificance for classifications of variable OB stars and supergiants,as opposed to mid-B stars. Another 10 yr of observations would berequired to derive the frequency spectra with an accuracy as high asthat available for white dwarfs.
| Photoelectric search for CP2-stars in open clusters. V - NGC 6281, NGC 6405, IC 4665
130 stars in the medium age clusters NGC 6281, NGC 6405, and IC 4665were observed by photoelectric photometry in the Delta a-system(Maitzen, 1976) in order to detect CP2-peculiarity by the presence ofthe lambda 5200 depression feature. No CP2-star was found in IC 4665,one in NGC 6281, and 3 in NGC 6405. Star No. 77 of NGC 6405 exhibits thestrongest lambda 5200 depression ever measured by this type ofphotometry. The (very small) reddening correction of the a-values isdiscussed and found to agree well with previous papers of this series.
| The Scorpio-Centaurus Association: II. Spectral types and luminosities of 220 O, B and a stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1961MNRAS.122..325M&db_key=AST
| Lichtelektrische Dreifarben-Photometrie von NGC 6405 (M6). Mit 3 Textabbildungen
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